CN107078656A - Voltage source converter and associated method - Google Patents

Abstract

Proposition is a kind of to include the voltage source converter of the first converter apparatus.First converter apparatus includes：Multiple guide valves, wherein each guide valve be included in provided in bridge for two anti-parallel thyristors switching between the two states, bridge is provided at least one phase leg, and phase leg is connected to two first pair of direct current poles and the midpoint including being connected to exchange AC terminal；And at least one reversing unit of the corresponding phase leg of at least one phase leg is connected to, wherein reversing unit is controllable, with the reverse bias IGCT when the IGCT of at least one guide valve of the phase leg of connection will stop conduction electric current.By providing anti-parallel thyristor in guide valve, bigger control is possible.For example, using guide valve, the electric current between DC poles can be turned off.

Description

Voltage source converter and associated method

technical field

The present invention relates to voltage source converters and associated methods.

Background technique

A high voltage direct current (HVDC) converter is mainly classified into a current source converter (CSC) and a voltage source converter (VSC). CSC converters are based on line-commutated converters (LCCs) containing thyristors. Thyristors have some inherent advantages such as being rugged, having low on-state losses, being easy to connect in series with each other, having a short time overrating capability and having high power rating and low cost. Also, thyristors are well established and commonly used components. However, LCC converters have several disadvantages: they can suffer from commutation failures, they require a strong alternating current (AC) grid, they do not support reactive power, have high harmonics, require polarity inversion for power inversion turn, leaving a large coverage area, the control is slow and does not improve AC grid stability.

VSC converters are based on self-commutated devices such as insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs), and others. These types of converters overcome many of the disadvantages of LCCs. It has the following characteristics: no commutation failure, it can be connected to weak AC grids (also passive loads), can support reactive power, has low harmonics, does not require polarity reversal for power reversal, control Fast and able to improve AC grid stability.

However, VSC converters have the following disadvantages: higher on-state losses, higher switching losses, increased rating (due to fault current), frequent failures and higher cost.

Recently, VSC converters employing multiple cells are also used, where each cell provides a voltage contribution to form the waveform used to obtain the AC signal.

These converters, also known as Modular Multi-Level Converters (M2LC), ie multi-cell VSC converters, have some additional advantages. They are available without filters. Furthermore, di/dt and dv/dt problems can also be avoided. However, they also provide a larger footprint due to the cell capacitors and the mentioned issues with VSC converters.

WO 2014/082657 A1 discloses a thyristor-based voltage source converter comprising a plurality of valves comprising switching elements with switching elements provided in a bridge for switching between two states, wherein the bridge is in at least one phase Provided in legs, the phase legs extend between two DC poles and have at least one midpoint connected to an AC terminal. The switching element of at least one of the valves is a thyristor, and the converter further comprises a commutation unit associated with the valve, wherein the commutation unit is controllable to reverse bias the valve if the valve is to stop conducting current.

However, improved fault handling is always of great benefit.

Contents of the invention

The object is to provide a voltage source converter with improved fault handling compared to the prior art.

According to a first aspect, a voltage source converter comprising first converter means is proposed. The first converter device comprises a plurality of pilot valves, wherein each pilot valve comprises two anti-parallel thyristors for switching between two states provided in a bridge provided in at least one phase leg, the phase leg connected to the two first pair of DC poles and including a midpoint connected to the AC terminal; and at least one commutation unit connected to a corresponding phase leg of the at least one phase leg, wherein the commutation unit is controllable to switch between The thyristor will stop conducting current when reverse biasing the thyristor connected to at least one of the phase legs of the pilot valve. Greater control is possible by providing anti-parallel thyristors in the pilot valve. For example, using a pilot valve, it is possible to shut off the current flow between the DC poles.

The voltage source converter may further comprise: a three-winding transformer; and second converter means comprising the mentioned features of the first converter means. In this case, for each phase, the AC terminals of the first converter device are connected to the first winding of the three-winding transformer, the AC terminals of the second converter device are connected to the second winding of the three-winding transformer, and the voltage source the AC terminals of the converter are connected to the third winding of the three-winding transformer; and each phase leg of the first converter means is connected in series with each phase leg of the second converter means between the main DC poles of the voltage source converter . By providing to the converter devices, the voltages from the two converter devices can be vector added on the AC side, allowing great control over the AC voltage and phase during normal and fault operation.

Each converter device may comprise at least three phases, and the voltage source converter may comprise three AC terminals, wherein for each phase, the AC terminal of the first converter device is connected to the first winding of the three-winding transformer, the second The AC terminals of the converter device are connected to the second winding of the three-winding transformer, and the AC terminals of the voltage source converter are connected to the third winding of the three-winding transformer.

The third winding may comprise, for each phase, a first winding portion and a second winding portion connected in series between the AC terminal and the counter terminal; and wherein the first winding portion is magnetically coupled to the first winding, and the second winding portion is magnetically coupled to the second winding.

The voltage source converter may further comprise a pilot valve configured to control the first converter device and the second converter device such that a desired voltage between the AC voltages of the first converter device and the second converter device is achieved. The control component of the differential phase angle.

Each converter device may comprise a wave shaper comprising a plurality of wave shaper units connected in series between the DC poles of the converter device, and wherein the midpoint of the wave shaper is connected to each commutation of the converter device units, wherein each wave shaper unit includes a switching element and an energy storage device.

Each converter arrangement may comprise interconnectors between each commutation unit and the phase leg to which it is connected, each interconnector comprising an anti-parallel thyristor.

Each commutation unit may comprise a full bridge unit.

Each wave shaper unit may comprise a full bridge unit.

Each wave shaper unit may comprise a half bridge unit.

The voltage source converter may further include a control component configured to detect a fault on the main DC pole; and to block the DC current by controlling the firing angle of the pilot valve to be greater than 90°, the blocking wave shaper unit and the blocking interconnector.

According to a second aspect, a method of controlling a voltage source converter comprising first converter means is proposed. The first converter device comprises a plurality of pilot valves, wherein each pilot valve comprises two anti-parallel thyristors for switching between two states provided in a bridge provided in at least one phase leg, the phase leg At least one commutation unit connected to the two first pair of DC poles and including a midpoint connected to the AC terminal; and a corresponding one of the at least one phase leg. The method comprises the step of controlling the commutation unit to reverse bias the thyristor of at least one pilot valve of a connected phase leg when the thyristor will stop conducting current.

The voltage source converter may further comprise a three-winding transformer and second converter means. The second converter device comprises the mentioned features of the first converter device, wherein for each phase the AC terminals of the first converter device are connected to the first winding of the three-winding transformer, the AC terminals of the second converter device connected to the second winding of the three-winding transformer, and the AC terminal of the voltage source converter is connected to the third winding of the three-winding transformer; and each phase leg of the first converter means is between the main DC poles of the voltage source converter connected in series with each phase leg of the second converter means. The method then further comprises the step of controlling the pilot valve of the first converter means and the pilot valve of the second converter means such that a desired differential between the AC voltages of the first converter means and the second converter means is achieved phase angle.

The method may further include the steps of: detecting a fault that is a short on all AC terminals; and handling the fault by controlling the pilot valve such that the differential phase angle is substantially 180 degrees for all phases. This can be achieved, for example, by controlling the pilot valves of the two stations and adjusting the DC link voltage.

The method may further comprise the steps of: detecting a fault that is a short circuit on only one of the AC terminals; and handling the fault by controlling the pilot valves of the first and second converter means such that the fault is counteracted. This can be achieved, for example, by controlling the pilot valves of the two stations and adjusting the DC link voltage.

The method may further comprise the steps of: detecting a fault on the main DC pole; and handling the fault by blocking the DC current by controlling the firing angle of the main bridge pilot valve greater than 90°, the blocking wave shaper unit and the blocking interconnector.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. Unless expressly stated otherwise, all references to "a/an/the element, device, component, part, step, etc." are to be construed in an open-ended manner as referring to at least one instance of the element, device, component, part, step, etc. . The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated otherwise.

Description of drawings

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a voltage source converter according to one embodiment of the invention;

Figures 2A-B illustrate voltage source converters according to two embodiments, each comprising two converter devices;

Figure 3 is a schematic graph illustrating the AC voltage generated by the voltage source converter of Figure 1 or 2;

Figure 4 is a schematic phasor diagram illustrating the resulting AC voltage generated by the voltage source converter of Figures 2A-B;

Figure 5 is a schematic diagram illustrating an embodiment of the transformer of Figure 2A;

Figure 6 is a schematic graph illustrating DC ripple in two modes of operation of the embodiment of Figures 1 and 2;

7A-C are schematic diagrams illustrating an embodiment of a converter unit of the converter of FIGS. 1 and 2; and

FIG. 8 is a flowchart illustrating a method performed in the voltage source converter of FIG. 1 or FIGS. 2A-B .

detailed description

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. However, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided as illustrations so that this disclosure will be thorough and complete, and will enable the present invention The scope of is fully conveyed to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig. 1 shows a converter 10 comprising a single converter device 11 according to a first embodiment of the invention. The converter arrangement 11 comprises a three-phase bridge consisting of a first phase leg 3a, a second phase leg 3b and a third phase leg 3c. More specifically, the phase legs 3a-c are connected between a first direct current (DC) pole P1 and a second DC pole P2. The first pole P1 is connected to the positive main DC pole 35, providing a first voltage U _DC ⁺ , and the second pole P2 is connected to the negative main DC pole 36, providing a second voltage U _DC ⁺ . Each of the first and second voltages U _DC ⁺ , U _DC ⁻ can be a positive voltage, a negative voltage or zero, as long as there is a difference between the voltages. The midpoints of the phase legs 3a-c are each connected, optionally via a respective reactor Xr, to a corresponding AC terminal TA, TB, TC, which in turn is connected to a winding of a transformer 13, in which case, as an example, The windings are delta connected. On the other side of the transformer 13 there are three AC terminals ACa, ACb, ACc connected to the converter means 11 , eg for connection to the windings of the AC grid.

The phase legs 3a-c comprise valves provided in the converter bridge. In the bridge there is a first valve V1 in the first half of the first phase leg 3a and a fourth valve V4 in the second half of the first phase leg 3a, where the midpoint of the first phase leg 3a is connected to the first AC terminal TA, forming the first AC phase. The second phase leg 3b comprises a third valve V3 in the first half and a sixth valve V6 in the second half, wherein the midpoint of the second phase leg 3b is connected to the second AC terminal TB, forming the second AC phase . In the bridge there is finally a fifth valve V5 in the first half of the third phase leg 3c and a second valve V2 in the second half of the third phase leg 3c, where the midpoint of the third phase leg 3c Connected to the third AC terminal TC, forming a third AC phase. Thus, a bridge is provided in at least one phase leg extending between at least two DC poles P1 , P2 and having at least one midpoint connected to an AC terminal.

Each valve, which may be called a pilot valve, is formed by using two anti-parallel thyristors. These valves are supplied in pairs. The upper half of the phase legs 3a-c are furthermore connected to the first pole P1 via an optional first filter 21a, while the lower half of the phase legs 3a-c are connected to the second DC pole via an optional second filter 21b. Pole P2. The phase legs 3a-c are connected in parallel between poles P1 and P2 in this way, where the connection to the first pole P1 is the first DC point marked X and the connection to the second pole P2 is the one marked Z Second DC point. The upper half of the phase legs 3a-c are all connected to the optional first filter 21a, and the lower half of the phase legs 3a-c are all connected to the optional second filter F2. There is also a wave shaper 14 comprising a plurality of wave shaper cells MLC1-6 connected in series between DC poles. Wave shaper units MLC1-6 together provide multiple voltage levels to provide a more sinusoidal AC waveform. The wave shaper unit is a converter unit and can eg be a full bridge unit (as shown), a half bridge unit (not shown) or a combination of full and half bridge units (not shown). The number of wave shaper units in Figure 1 is an example only and more or fewer wave shaper units could be used.

The midpoint of the wave shaper 14 is connected to the midpoint of each phase leg 3a-c of the bridge via a corresponding intermediate branch. The midpoint is furthermore marked with a Y, which forms the third DC point. Thus, there is here a first intermediate branch interconnecting the midpoint of the first phase leg 3a with the midpoint of the wave shaper 14 and a midpoint of the second phase leg 3b with the midpoint of the wave shaper 14. A second intermediate branch and a third intermediate branch interconnecting the midpoint of the third phase leg 3 c with the midpoint of the wave shaper 14 . Each intermediate branch includes a bidirectional switch and a commutation unit. The first intermediate branch thus comprises a first bidirectional switch SWA and a first commutation unit CCA, the second central branch comprises a second bidirectional switch SWB and a second commutation unit CCB, and the third central branch comprises a third bidirectional switch SWC and a first commutation unit CCA. Three-commutation unit CCC.

Furthermore, there is a control unit 12 which controls the operation of the converter device 11 . In FIG. 1 , the control paths from the control components are not shown so as not to unnecessarily complicate FIG. 1 . The control part 12 controls the second reversing unit as all pilot valves, bidirectional switches and multi-stage units both in normal operation and in fault handling.

The filters 21a-b can be provided, for example, in the form of full-bridge units, which can be provided for filtering harmonics, for example.

As mentioned above, each valve uses anti-parallel thyristors as switching elements. Furthermore, the bidirectional switches SWA, SWB, SWC are provided as anti-parallel thyristors. Optionally, each bidirectional switch includes two pairs of antiparallel thyristors. As an alternative, these bidirectional switches SWA, SWB, SWC can also be provided by using transistors such as IGBTs.

The operation of the converter arrangement 11 will now be described with combined reference to FIG. 3 showing one cycle of three AC voltages supplied by the three phase terminals of the converter arrangement 11 . Operation will be described for one of the phases, for example the first phase A, and the voltage for this phase has portions, a first portion 14 between 0 and 30 degrees, a second portion 16 between 30 and 150 degrees, a third portion 18 is between 150 and 210 degrees, the fourth portion 20 is between 210 and 330 degrees, and the fifth portion 22 is between 330 and 360 degrees. The fifth part 22 will continue with the first part of the following cycle.

From the second part 16, the first valve V1 will conduct, which will put the first AC terminal at a high voltage level. Through operation of the wave shaper 14, this high voltage will change. More specifically, the wave shaper 14 is controlled by the control unit 12 so that the voltage is gradually changed from half U _DC ⁺ /2 of the maximum positive voltage to the maximum positive voltage U _DC ⁺ and back between 30 and 150,50 degrees to half U _DC ⁺ /2 of the maximum voltage. Afterwards, the first valve V1 stops conducting, followed by the conduction of the first intermediate branch between 150 and 210 degrees so as to form the third section 18, wherein the first intermediate branch will provide substantially zero voltage, by using the wave shaper 14, the The voltage changes from half U _DC ⁺ /2 of the maximum positive voltage to half U _DC /2 of the maximum negative voltage.

Afterwards, the fourth valve V4 will start conducting, which will put the first AC terminal TA at a low voltage level. Through operation of the wave shaper 14, this low voltage will change. More specifically, the wave shaper 14 is controlled such that the voltage changes from half U _DC ^- /2 of the maximum negative voltage to U _DC ^- of the maximum negative voltage, and back to half U of the maximum negative voltage between 210 and 330 degrees _DC ⁺ /2 to form the fourth part 20. Afterwards, the fourth valve V4 is closed, and likewise this is followed by the conduction of the first intermediate branch, which takes place between 330 degrees of the first cycle and 30 degrees of the following cycle in order to provide the fifth part 22 and the following cycle first part. The bidirectional switch then provides essentially zero voltage, which is changed via zero voltage from half U _DC ⁻ /2 of the maximum negative voltage to half U _DC ⁺ /2 of the maximum positive voltage using the wave shaper 14 to provide the fifth portion 22 voltage. As can be understood from what has been described above, this operation is also valid for the first part 14 of the first cycle. Furthermore it can be seen that for each phase the bridge of the valve switches between two different states, with the middle branch providing the intermediate states and the wave shaper 14 providing the voltage changes associated with each state to form the AC waveform.

The same type of operation is provided for the other phases in order to obtain the three-phase voltage shown in FIG. 3 . This means that when the first valve V1 stops conducting at 150 degrees, then the third valve V3 of the second branch conducts between 150 and 270 degrees, followed by the fifth valve V5 between 270 degrees and 30 degrees of the following cycle conduction. In the same way, the conduction of the fourth valve V4 is preceded by the conduction of the second valve V2 between 90 and 210 degrees, and is followed by the conduction of the sixth valve between 330 degrees and 90 degrees of the following cycle.

As can thus be seen, each phase A, B, C is connected at each DC point X, Y, Z for a duration of 120° as indicated above. At any moment, the three phases are connected to one of the three DC points X, Y, Z, and the three-phase bridge operates as a 120° mode of operation. When the two phase voltages are equal, the phase points are swapped between DC points. The wave shaper 14 operates with a sinusoidal waveform for each phase. The upper and lower halves of the phase legs operate complementary, and thus it is possible to obtain three-phase voltages with a small number of switching operations.

What has been described above is the general operation of a converter of the type shown in FIG. 1 . However, valves and bi-directional switches include thyristors. As is well known, thyristors are easily turned on. However, it cannot be turned off unless it is subjected to a negative voltage. Commutation units CCA, CCB, CCC are provided for this purpose, ie to provide a negative voltage across the thyristors to thereby switch them off. In this way it is then possible to operate the thyristors as fully controllable switching elements such as insulated gate bipolar transistors (IGBTs) and thus operate the converter as a voltage source converter.

As can be seen from the description made above, a thyristor-based VSC comprises a thyristor-based three-phase bridge, a string of wave shaper units connected in series, and a VSC connecting the midpoint of the three-phase bridge to the midpoint of the wave shaper. The middle branch of the bidirectional thyristor. In this first embodiment a commutation unit is added for each phase in the middle branch to commutate the thyristors. The full-bridge unit can be used as a commutation unit because it provides three-level voltage. During phase crossing, the commutation unit supplies a negative voltage across the thyristors, and it supplies a small negative current for thyristor commutation. Valves and bi-directional switches operate at the fundamental frequency. Anti-parallel thyristors are used as switching devices for valves and bi-directional switches in order to benefit from the advantages of this type of component. To commutate the thyristors a commutation unit is used. To commutate a valve comprising a thyristor, a reversing unit reverse biases the valve. The commutation unit is thus associated with at least one valve and is controllable to reverse bias it by providing a negative voltage across the thyristor of this valve if it should stop conducting current. Here, the negative voltage is a voltage that supplies a current having a direction opposite to the current conduction direction of the thyristor.

Since the valves comprise anti-parallel thyristors and the overall structure of the converter device 11 is symmetrical, the DC poles can be reversed without any physical reconfiguration. This can be used, for example, for power inversion.

Furthermore, the firing angle of the thyristors in the valve can be controlled to obtain a reduced DC voltage, for example during a DC voltage increase in DC fault recovery.

Figures 2A-B illustrate voltage source converters according to two embodiments, each comprising two converter arrangements. Referring first to FIG. 2A , the voltage source converter 10 includes first converter means 11 and second converter means 11 ′. Each converter device 11 , 11 ′ is a converter device as described above with reference to FIG. 1 .

Furthermore, there is a three-winding transformer 13 . For each phase, the AC terminals TA, TB, TC of the first converter device 11 are connected to the first winding 30 of the three-winding transformer 13 . Similarly, for each phase, the respective AC terminals TA′, TB′, TC′ of the second converter means 11 ′ are connected to the second winding 31 of the three-winding transformer 13 .

Furthermore, for each phase, the AC terminals ACa, ACb, ACc of the voltage source converter device 10 are connected to the third winding 32 of the three-winding transformer 13 .

The two converter devices 11 , 11 ′ are connected in series between the main DC poles 35 , 36 . Each phase leg of the first converter means 11 is thus connected in series with each phase leg of the second converter means 11 ′ between the main DC poles 35 , 36 of the voltage source converter 10 . Compared to the converter device 11 of Fig. 1, each converter device 11, 11' of Fig. 2A need only have half the rating.

A general control unit 17 controls the operation of the two converter devices 11, 11'.

Turning now to Figure 2B, for each phase, the third winding 32 of the three-winding transformer 13 here comprises a first winding portion 33a and a second winding portion 33b. These winding parts 33a-b are connected in series between the AC terminal and the counter terminal. The AC terminal can eg be one of the AC terminals ACa, ACb, ACc eg for connection to the grid. The counter terminal can eg be the other terminal of the AC terminals ACa, ACb, ACc leading to a delta connection, or ground leading to a star connection. The first winding portion 33a is magnetically coupled to the first winding 30 and the second winding portion 33b is magnetically coupled to the second winding 31 .

In the embodiment of Fig. 2A, a common core is used to magnetically add the contribution from the converter means 11, 11' to the AC side. In the embodiment of Fig. 2B, the contributions from the converter means 11, 11' are added electrically using the same current.

Referring now to FIG. 4, as shown for example in FIGS. The voltage of is the result of the vector addition of the corresponding voltages from the two converter devices. Looking at Fig. 4, for phase A the voltage vector Va at the AC terminal ACa is the result of the vector addition of the voltage vector Va1 of the first converter means 11 and the voltage vector Va2 of the second converter means 11'.

Using the two vector additions possible due to the two converter devices, it is possible to control P (active power) and Q (reactive power) independently. This is achieved by the overall control unit 17 controlling the differential phase angle Θ between the two component voltages Va1, Va2. In addition, since the magnitude of Va can be controlled between 0 and |Va1|+|Va2| by changing the phase shift Θ, the AC voltage and DC voltage are decoupled.

FIG. 5 is a schematic diagram illustrating an embodiment of the transformer of FIGS. 1 and 2 . Here, the converter side is connected to the first converter 11 and optionally in the same way (but in a separate winding) to the second converter 11'. The converter sides are arranged in a triangle pattern. This means that each phase terminal TA, TB, TC (TA', TB', TC') is separated from the other phase terminals via a winding and there is no neutral point. The AC port side is connected to AC terminals ACa, ACb, ACc, and connected in a star pattern. This means that each phase is connected to the neutral point N via a winding. Optionally, the neutral point is connected to ground.

Fault handling will now be described in somewhat more detail.

One fault condition is an LLLG fault where all AC terminals are shorted. In this case, all three lines on the grid are shorted to ground. In the embodiment of Figures 2A-B, by setting a 180 degree phase shift, the magnitude of Va can be set to zero (or substantially zero) to isolate the fault on the AC side.

Another fault case is an LG fault in which one line on the grid side is short-circuited to ground. This fault case will now be analyzed using sequence components in the case where the first phase A is short-circuited to ground. Equivalent analysis can be applied to the second phase B and the third phase C.

The grid-side voltage contains positive, negative and zero-sequence components. Sequence components can be derived by the following transformations.

Among them, U ⁺ is the positive sequence voltage component, U ^- is the negative sequence voltage component, and U ⁰ is the positive sequence voltage component, and a is , that is, the phase with magnitude 1 and phase angle 120 ˚.

Under the LG fault, the above equation can be written as:

Solving the above equation,

,and

therefore,

When the transformer is in a delta-wye configuration, zero-sequence voltage does not appear on the converter side. Therefore, the converter side voltage can be expressed as follows.

These voltages can be generated by using the first converter means 11 to generate positive sequence voltages and using the second converter means 11' to generate negative sequence voltages. Note, however, that this technique will require an increased rating of the lower converter (0.66 U _m instead of 0.5 U _m ). In other words, the voltage of (6) is achieved by controlling the pilot valve, counteracting the LG fault.

Another fault situation is a fault on the DC side, where two terminals on the DC side are short-circuited. In this case, the control part 12 and/or 17 controls the bidirectional switches SWA, SWB, SWC and the wave shaper units MLC1-6 into blocking mode upon detection of the occurrence of a dc-side fault. The three-phase bridge operates at firing angles higher than 90°. In this way, the energy in the line inductance (defined as the inductance between the converter 10 and the fault location) is fed back to the AC and the fault current rapidly decays. The voltage in the energy storage device in the wave shaper unit counteracts the trapped current. When the switching element is blocked, the capacitor of the wave shaper unit counteracts the trapped current. At that time, the current in the fault loop flows through the anti-parallel diode (of the wave shaper cell) and prevents the cell capacitor voltage. In this way, the trap current is kept low.

FIG. 6 is a schematic graph illustrating DC ripple in two modes of operation of the embodiment of FIGS. 1 and 2 . When operating this converter, 6n harmonics may appear at the DC side. As shown in Figure 1, filters 21a-b at the dc side can be used to reduce these harmonics.

Furthermore, by employing a 30 degree phase shift between the voltages of the wave shapers 14 of the up-converter and down-converter when using one of the embodiments of Figures 2A-B, further reduction in ripple can be achieved. The valves of the two converter means 11, 11' are pulsed every 30 degrees with a phase shift of 30 degrees compared to every 60 degrees with zero phase shift, which is why the ripple is reduced with a phase shift of 30 degrees.

As seen in Figure 6, the voltage 38 on the DC side where a 30 degree phase shift is applied exhibits substantially reduced ripple compared to the voltage 39 on the DC side where no phase shift is applied.

7A-C are schematic diagrams illustrating an embodiment of a converter unit of the converter of FIGS. 1 and 2 . Each of the converter units 2 can be used for wave former units MLC1-6 and commutation units CCA-C as described above.

Fig. 7A illustrates a converter unit 2 comprising a switching element 40 and an energy storage element 41 in the form of a capacitor. The switching element 40 can eg be realized using an insulated gate bipolar transistor (IGBT), an integrated gate commutated thyristor (IGCT), a gate turn-off thyristor (GTO) or any other suitable high power semiconductor component. In fact, the converter unit 2 of Fig. 7A can be regarded as a more general representation of the converter unit shown in Fig. 7B to be described here next.

Fig. 7B illustrates a converter unit 2 implementing a half bridge structure. The converter unit 2 here comprises the legs of two series-connected switching elements 40a-b, for example in the form of IGBTs, IGCTs, GTOs or the like. Optionally, there are anti-parallel diodes connected across each switching element 40a-b (not shown). An energy storage element 41 is also provided in parallel with the legs of the switching elements 40a-b. The voltage synthesized by the converter unit 2 can thus be zero or the voltage of the energy storage element 41 .

Fig. 7C illustrates a converter unit 2 implementing a full bridge structure. The converter unit 2 here comprises four switching elements 40a-d, eg IGBT, IGCT, GTO etc. Optionally, there are anti-parallel diodes connected across each switching element 40a-d (not shown). An energy storage element 41 is also provided in parallel across the first legs of the two switching elements 40a-b and the second legs of the two switching elements 40c-d. Compared to the half-bridge of FIG. 7B , the full-bridge structure allows the synthesis of voltages that can assume two signs, whereby the voltage of the converter unit can be 0, the voltage of the energy storage element 41 or the reverse voltage of the energy storage element 41 .

FIG. 8 is a flowchart illustrating a method performed in the voltage source converter of FIG. 1 or FIGS. 2A-B .

In the control commutation unit step 50, the control commutation unit CCA, CCB, CCC reverse biases the thyristor of at least one pilot valve of a connected phase leg 3a-c when the thyristor will stop conducting current.

In the control pilot valve step 52, the pilot valve of the first converter device and the pilot valve of the second converter device are controlled such that the desired AC voltage between the first converter device and the second converter device is achieved. Differential phase angle.

In a condition detect fault step 54 it is checked whether a fault can be detected. The fault can be, for example, a LLLG fault, a LG fault or a DC fault.

In the process fault step 56, appropriate measures for the fault are taken.

For example, by controlling the firing angle of the pilot valves V1-V6 to be greater than 90°, the wave shaper unit and the blocking interconnector are blocked to block DC current when a DC fault is detected.

When the detected fault is a short circuit on all AC terminals, ie a LLLG fault, the pilot valve is controlled such that the differential phase angle is substantially 180 degrees for all phases.

When the detected fault is a short circuit on only one AC terminal, ie LG, the pilot valve is controlled such that the fault is counteracted, for example by applying the voltage of equation (6).

It is to be noted that while the embodiments shown herein refer to three phases, the same principles can be applied to any number of phases.

The invention has been described above mainly with reference to a few embodiments. However, as will be readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims (16)

1. A voltage source converter (10) comprising first converter means (11), comprising: A plurality of pilot valves (V1, V2, V3, V4, V5, V6) where each pilot valve comprises two anti-parallel thyristors for switching between two states provided in a bridge at least provided in one phase leg (3a-c) connected to the two first pair of DC poles (P1, P2) and comprising a midpoint connected to the AC terminals (TA, TB, TC); and at least one commutation unit (CCA, CCB, CCC) connected to a respective phase leg of said at least one phase leg (3a-c), wherein said commutation unit (CCA, CCB, CCC) is controllable, To reverse bias the thyristor of at least one pilot valve of the connected phase leg (3a-c) when said thyristor will stop conducting current. 2. The voltage source converter (10) of claim 1, further comprising: Three-winding transformer (13); and second converter means (11') comprising the mentioned features of said first converter means (11); wherein for each phase said AC terminals of said first converter means (11) are connected to a first winding of said three-winding transformer (13), said second converter means (11') The AC terminals are connected to the second winding of the three-winding transformer (13), and the AC terminals (ACa, ACb, ACc) of the voltage source converter (11) are connected to the third winding of the three-winding transformer (13). windings; and Each phase leg of said first converter means (11) is connected in series with each phase leg of said second converter means (11') between the main DC poles of said voltage source converter (10) . 3. A voltage source converter (10) as claimed in claim 2, wherein each converter means (11, 11') comprises at least three phases, said voltage source converter (10) comprising three AC terminals, wherein for each phase said AC terminals of said first converter means (11) are connected to a first winding (30) of said three-winding transformer (13), said second converter means (11') Said AC terminal of said three-winding transformer (13) is connected to the second winding (31) of said three-winding transformer (13), and said AC terminal of said voltage source converter (10) is connected to the second winding (31) of said three-winding transformer (13) Three windings (32). 4. A voltage source converter (10) as claimed in claim 2 or 3, wherein said third winding (32) comprises, for each phase, a first winding portion connected in series between the AC terminal and the counter terminal ( 33a) and a second winding portion (33b); and wherein said first winding portion (33a) is magnetically coupled to said first winding (30), and said second winding portion (33b) is magnetically coupled to said first Second winding (31). 5. A voltage source converter (10) according to any one of claims 2 to 3, further comprising a pilot valve configured to control said first converter means (11) and said second converter means (11 ') of the pilot valve so as to achieve the desired differential phase angle between the AC voltage of the first converter means and the control means (17) of the second converter means. 6. A voltage source converter (10) as claimed in any one of the preceding claims, wherein each converter device (11, 11') comprises a wave shaper (14) comprised in said converter device (11 , 11') between a plurality of wave shaper units connected in series between said DC poles (P1, P2), and wherein the midpoint of said wave shaper (14) is connected to said converter means (11, 11 ') of each commutation unit (CCA, CCB, CCC), wherein each wave shaper unit includes a switching element (40, 40a-d) and an energy storage device (41). 7. A voltage source converter (10) as claimed in any one of the preceding claims, wherein each converter arrangement comprises an interconnector ( SWA, SWB, SWC), each interconnector includes anti-parallel thyristors. 8. A voltage source converter (10) as claimed in any one of the preceding claims, wherein each commutation unit (CCA, CCB, CCC) comprises a full bridge unit. 9. A voltage source converter (10) as claimed in any one of the preceding claims, wherein each wave shaper unit comprises a full bridge unit. 10. A voltage source converter (10) as claimed in any one of the preceding claims, wherein each wave shaper unit comprises a half bridge unit. 11. A voltage source converter (10) according to any one of the preceding claims, further comprising control means (12, 17) configured to detect a voltage on said main DC poles (U _DC ⁺ , U _DC ⁻ ) failure; and blocking the DC current by controlling the firing angle of the pilot valves (V1-V6) greater than 90°, blocking the wave shaper unit and blocking the interconnector. 12. A method of controlling a voltage source converter (10) comprising first converter means (11) comprising: a plurality of pilot valves (V1, V2, V3, V4, V5, V6, SWA, SWB, SWC), where each pilot valve comprises two anti-parallel thyristors for switching between two states provided in a bridge in at least one phase leg (3a-c ), said phase leg is connected to two first pair of DC poles (P1, P2) and includes a midpoint connected to an alternating current AC terminal (TA, TB, TC); and connected to said at least one phase leg ( 3a-c) of at least one commutation unit (CCA, CCB, CCC) of the corresponding phase leg, said method comprising the steps of: The control (50) of the commutation unit (CCA, CCB, CCC) reverse biases the thyristor of at least one pilot valve of a connected phase leg (3a-c) when said thyristor will stop conducting current. 13. The method of claim 12, wherein said voltage source converter (10) further comprises a three-winding transformer (13); and second converter means (11') comprises said first converter means (11 ), wherein for each phase said AC terminals of said first converter means (10) are connected to a first winding of said three-winding transformer (13), said second converter The AC terminal of the device (10') is connected to the second winding of the three-winding transformer (13), and the AC terminal of the voltage source converter (10) is connected to the second winding of the three-winding transformer (13). three windings; and each phase leg of said first converter means (11) is connected to each phase leg of said second converter means (11') between the main DC poles of said voltage source converter (10) The phase legs are connected in series, wherein the method further comprises the steps of: controlling (52) a pilot valve of said first converter means and a pilot valve of said second converter means such that an AC voltage between said first converter means and said second converter means is achieved expected differential phase angle. 14. The method of claim 13, further comprising the steps of: Detect faults (54) that are shorts on all AC terminals (TA, TB, TC); and Said fault is handled by controlling said pilot valves of said first converter means (11) and said second converter means (11') such that said differential phase angle is substantially 180 degrees for all phases ( 56). 15. The method of claim 13, further comprising the steps of: detecting a fault (54) that is a short circuit on only one of said AC terminals (TA, TB, TC); and Said fault is handled (56) by controlling said pilot valves of said first converter device (11) and said second converter device (11') such that said fault is counteracted. 16. A method as claimed in claim 12 or 13, further comprising the steps of: detecting a fault (54) on said main DC pole; and Said fault is handled by controlling said firing angle of said pilot valves (V1-V6) to be greater than 90°, blocking said wave shaper unit and blocking said interconnector to block DC current (56).